Im not sure, but, you change the audio output in audio preferences? (Playback devices) You need use the digital audio (S-PDIF).
Yes is very basic, but is for sure 🙂 And other way, you can make this change on your phoebus drivers menú. In spdif out.
When you use the digital output, you have two options, send the signal on pure raw, no decoded to the channels. Or send the decode
signal to your external deco. I supposed your use this option.
Check carefully the cables. The audio settings, sample rate, etc...on your audio card drivers.
Sorry me english, cheers!